The present invention relates to a method for monitoring cracks in a sample of materials by using phase angle, and the method for monitoring cracks by using phase angle can be used to determine the critical concentration of a solution, the concentration being capable of forming cracks in a sample of materials.
Materials always have the possibility of forming cracks, for example, some materials may demonstrate sensitization in the welding process. If a corrosive ion such as chloride ion solution is added at this time, the materials will be corroded and form cracks.
An European Patent No. 70124 describes a method of detecting and quantifying damage in metal structures by using the strain electrometric technique to measure the local surface potential of a specimen. However, this method must require an electrolytic cell wherein, as one electrode, the metal structure to be monitored and at least one reference electrode are at least partially immersed in an electrode having properties which cause passivation of the immersed surface of the metal structure and at least partial de-passivated thereof when a flow of current to the passivated surface is established, and once the surface has become initially passivated, detecting any change in the electrochemical potential of the immersed metal structure. The passivation characteristics of the monitored metal must be known before applying this method, thus this causes some inconveniences in application.
A U.S. Pat. No. 4,048,558 describes a method and apparatus for detecting metal failures in situ. The method comprises making measurements of an electrical characteristics of a circuit including a section of the metal structure at different applied frequencies. Through such measurement, it is possible to detect cracks and follow their progress as they deepen. Because of skin effect, the impedance change resulting from cracks in the surface of the metal will be detected, and by monitoring the impedance at each of these frequencies the depth of a crack can be determined as long as it has a transverse component. However, in order to measure the impedance change, the metal to be tested must be welding a signal receiving point. In addition, the environment of the metal to be tested, for example, temperature and the flow speed of the liquid can affect the response of the signals.
A French Patent No. 2,274,042 describes a method for monitoring the defective metal. This method uses a probe with two impedance settings connected on an AC impedance bridge, and the impedance is changed to generate an output potential change, so as to monitor the defective metal. However, the probe must be applied to the method, the probe may be corroded by the testing metal having corrosive properties and affect the accuracy of the output potential.
A U.S. Pat. No. 4,307,610 describes a method for measuring crack propagation in samples with a high frequency pulsator, but this method uses only in load cycle cases.